The present invention relates to improved methods and apparatus for delivering radio frequency (RF) power for RF power process operations.
RF power is used in a wide variety of applications for carrying out process operations. Exemplary of such process operations is the use of RF induction power for heating. RF induction heating involves coupling RF power to a material, such as a workpiece, that absorbs the RF power and converts the RF power into heat. In other words, the currents induced in material by the RF power are converted into heat because of the electrical resistance of the material that absorbs the RF power. In this manner, the RF power can be used to heat the workpiece without having physical contact between the power source and the object. This type of heating can be used when the workpiece is the material that absorbs the RF power, and the workpiece is heated directly by the RF power. Alternatively, the workpiece may be in contact with or near a second material that absorbs RF power. The second material absorbs the RF power and creates heat. The heat is then transferred to the workpiece by conduction, convection, or radiation.
In another example of RF heating, the RF power can be coupled to a gas to produce a thermal plasma. Free electrons in the thermal plasma absorb the RF power and they are raised to high energy levels. These energetic free electrons interact with other gas phase species to produce a high temperature mixture capable of transferring thermal energy to other gases, liquids, or solids.
The thermal plasmas mentioned above can be used to promote chemical reactions. Chemical reactions can be promoted because of the high temperatures of the thermal plasma. Alternatively, thermal plasmas are able to promote chemical reactions because of the ability of the energetic electrons to break chemical bonds and allow chemical reactions to occur that would proceed with difficulty under non-plasma conditions.
The manufacture of optical fiber pre-forms is an example of the use of thermal plasmas generated using RF power. The RF thermal plasma provides the energy for driving the chemical reactions in gas mixtures of silicon compounds, oxygen, and dopants. The chemical reactions cause deposition of doped silica layers.
Another example involving RF power thermal plasmas is the operation of high-pressure gas lasers. In gas laser operation, the important characteristic of the RF plasma is the light emission that occurs because of the plasma. The thermal energy that is produced is generally not considered important to the operation of the laser.
In other applications, RF power is used to produce non-thermal plasmas, also referred to as non-equilibrium plasmas. The fabrication of semiconductor devices is one area in which non-thermal plasmas are extensively used. The non-thermal plasmas are used for etch processes wherein the non-thermal plasmas are used to generate reactive species in a gas to accelerate reactions between the species and a solid surface. The etch process can be a general removal of components on the surface as in a cleaning process or the selective removal of material from certain areas on the surface through use of a masking material that has been previously patterned. Non-thermal plasmas are used to promote deposition reactions wherein gas phase species are caused to react to form a solid product that deposits on surfaces. During the manufacture of semiconductor devices, etch processes involving RF plasmas and deposition processes involving RF plasmas are used repeatedly during the fabrication process. One of the main benefits of using the non-thermal plasma is the ability of the non-thermal plasma to stimulate chemical reactions that would otherwise require temperatures that are too high for use in the fabrication of semiconductor devices.
RF power non-thermal plasmas are also used as cleaning processes in the fabrication of semiconductor devices. The non-thermal plasmas are commonly used to strip photoresist materials from semiconductor wafers as part of post etch wafer clean procedures. The photoresist material serves as a mask material during etch processes used in patterning the surface of the devices. Resist material is stripped from the surface of the wafers by creating a non-thermal plasma in a gas containing oxidizing species such as oxygen and possibly halogen species that are capable of reacting with and volatilizing the resist material. In some applications, the non-thermal plasma is maintained at a position upstream of the wafer. Reactive species generated in the non-thermal plasma flow downstream and react with the wafer surface for the stripping process.
Another cleaning process that uses non-thermal plasmas is the cleaning of reaction chambers used in semiconductor manufacturing. Sometimes, the reaction chambers used in plasma etch processes experience a buildup of deposits from the etch process. These deposits need to be removed as part of the reactor maintenance process. In addition, the reactors that are used in deposition processes for semiconductor device fabrication undergo a buildup of deposits on the reactor walls; the wall deposit must be removed as part of reactor maintenance. Non-thermal plasmas generated using RF power and gases containing species that are reactive with the deposits have been used to volatilize and removed the deposits built up on the walls of etch reactors and deposition reactors.
RF power plasmas have also been used for decomposition of chemical compounds that are hazardous or otherwise undesirable. Some of the compounds are highly refractory in nature and are difficult to decompose. Examples of compounds that have been decomposed or abated with plasmas include chlorofluorocarbons (CFC) and perfluorocompounds (PFC).
The applications given above where RF power is used as part of a process makeup only a small fraction of the applications for RF power. There are numerous additional processing applications for RF power. However, the methods and apparatus typically used to deliver RF power have deficiencies and may be inefficient for use in some RF power process operations. Some of the deficiencies are common for multiple applications. The existing deficiencies in the prior methods and apparatus for RF power delivery may limit the use of RF power for possible new applications.
One frequently encountered problem with prior RF power delivery systems is that the equipment tends to be large and heavy. There are instances in which the size of the RF power generator greatly exceeds the size of the processing chamber. Problems resulting from the large size of the equipment include taking up excess space on a factory floor. The excess space required by the equipment can be quite expensive if it is in a high-cost factory, such as a cleanroom used in semiconductor manufacturing. The large size also makes transporting the equipment difficult. Moving the apparatus frequently requires more than one person and the use of moving equipment.
A second problem with existing RF power delivery systems is their complexity. The existing systems frequently have redundant systems and extra capabilities that are unnecessary. In addition, the effort to derive data for controlling the RF power delivery is unnecessarily complex.
Here is one example of how a typical old-style RF power delivery system operates. Low frequency AC power is rectified and then switched to provide current to the RF amplifier. The RF amplifier drives current through an output match network and then through an RF power measurement circuit to the output of the power supply. The output match is usually designed to provide RF power that matches an impedance of 50 ohms. The 50 ohm impedance match is necessary in order to have the same characteristic impedance as the industry standard coaxial cables. Power flow through the 50 ohm coaxial cable section is measured again by a load match controller. The instrument used for measuring the power is also designed to be compatible with the 50 ohm impedance of the coaxial cable. A load match, usually a variable RF match with a motorized automatic tuner, transforms the RF power again to enable the RF power to be coupled to a load. Motors in the tuner can change the values of variable capacitors and inductors present in the tuner. The variable load match is necessary in order to accommodate changes in the impedance of the load. Specifically, the variable load match makes it possible for the RF amplifier to provide RF power at a constant impedance to a load that may have a variable impedance. This arrangement allows constant RF power delivery if the load impedance changes.
In the example just presented, the RF power is measured multiple times at different points between the RF amplifier and the load. In addition, special coaxial cable and measuring instruments are necessary in order to comply with the industry standards. Furthermore, the RF power measurements typically include forward RF power measurements and reflected RF power measurements.
The coaxial cables described in the previous example present an additional problem for high frequency RF power. The length of the coaxial cable may be long enough, with respect to the wavelength of the RF power, to allow standing wave formation from the forward RF power and reflected RF power. Standing waves in the coaxial cable produce high peak currents and voltages that can damage the coaxial cable.
For low frequency RF power delivery systems, the operation may be different from that of the previous example. The low frequency RF power delivery systems use a low frequency RF power amplifier. The low frequency RF amplifier is coupled through a fixed load match to a load. Because the RF frequency is low, the transmission line between the RF amplifier and the load generally is much shorter than a quarter wavelength, so that no standing wave pattern occurs in the coaxial cable. Consequently, there are no problems with high peak voltages and peak currents for low frequency RF power delivery systems.
In the typical old-style RF power delivery systems, whether low frequency or high frequency, forward RF power and reflected RF power are both measured. Usually the forward and reflected RF power measurements are made with a device such as a dual directional coupler. The forward and reflected power measurements are used as part of the RF power control system to maintain a constant power delivery to the load. Measurements of the reflected RF power is used in high frequency RF power delivery systems to control the variable load match such that match conditions are obtained that give a substantially zero reflected power. In low frequency RF power systems, the reflected power measurements are used to adjust the forward RF power to obtain the desired net RF power delivery to the load.
Clearly, there are numerous applications requiring RF power delivery systems. Unfortunately, typical methods and apparatus for old-style RF power delivery systems have characteristics that may be undesirable for some applications. There is a need for RF power delivery methods and apparatus that are simple in operation, use a minimum number of parts, minimize redundancy, and provide high reliability.